Commercial and residential water heaters typically heat water by generating tens of thousands, and even hundreds of thousands, of BTUs. For many years, manufacturers of water heaters, and especially manufacturers of water heaters to be used in commercial applications, have sought to increase the efficiency of the exchange of this heat energy from burned fuel to the water contained in the water heater. Accordingly, maximized heat exchange efficiency has long been an object of commercial and residential water heater manufactures.
As heat exchange efficiency increases, however, such increased efficiency gives rise to the problems associated with condensation of water vapor from the products of combustion. More specifically, upon burning of a mixture of fuel and air, water is formed as a constituent of the products of combustion. It is recognized that as the temperatures of the combustion gases decrease as the result of successful exchange of heat from the combustion gases to water in the water heater, the water vapor within the combustion gases tends to be condensed in greater quantities. In other words, as the temperatures of the combustion gases decrease as a direct result of increasingly efficient exchange of heat energy to the circulated water, the amount of condensate forming on the heat exchange surfaces also increases. This condensate is typically found to increase when heat exchange efficiencies exceeding about 90% are achieved.
For example, U.S. Pat. No. 7,559,293 discloses a high efficiency water heater having a flue system designed to provide improved heat exchange efficiency. The flue system includes an upstream heat exchange portion having at least one substantially vertical flue tube. The flue system also includes a downstream heat exchange portion having at least one substantially vertical flue tube. The upstream heat exchange portion provides a first pass for heat exchange with water in a water heater. The downstream heat exchange portion provides a second pass for heat exchange with water in the water heater. Condensate may form in the heat exchange tubing (or flues) as the efficiency of heat exchange increases by virtue of the reduced temperature of the exhaust gases. In order to manage any such condensation, the water heater is optionally provided with a condensate drain or a condensate pump or other means for permitting the condensate to flow or be withdrawn from the water heater.
Commercial and residential water heaters can be designed to operate below the efficiencies at which increased quantities of condensate are likely to form (i.e., below the condensing mode). To do so, however, compromises the efficiency of the water heater. Accordingly, there continues to be a need for improved water heating systems having targeted heat exchange efficiency yet resisting the effects of water vapor condensation associated with such efficiency.